Printed circuit boards commonly have protection from chemical attack and mechanical damage which may occur during portions of their processing, or in subsequent use. For rigid printed circuit boards protection is often provided by a polymer coating or covering, for example, an epoxy or polyimide polymer, applied to the exposed surface.
For flexible printed circuit boards the protective material has the additional requirement that it be capable of being flexed without loss of adhesion or cracking. Protection in these cases is frequently provided by composite films known as coverlay films. These coverlay films generally consist of a tough, flexible, non-porous plastic film having a layer of flexible adhesive on it. The non-porous plastic film used in a coverlay film is often a polyester polymer film or a film of a polyimide polymer, such as Kapton.RTM. or Apical.RTM..
The adhesive used in a coverlay film is typically a synthetic polymer material having a low glass transition temperature (Tg), such as an acrylic elastomer resin. While these adhesive materials provide good flexibility and room temperature adhesion, they have limited chemical resistance, poor thermal performance, poor moisture resistance, and are not inherently self-extinguishing. Additionally, they have relatively high dielectric constants (Er) compared to the dielectric materials used in printed circuit boards for high frequency service. The adhesive of the coverlay must be sufficiently thick, typically more than 20 micrometers, in order to encapsulate and protect the printed electrical conductors and circuitry on the surface of the printed circuit board. The amount of adhesive required can raise the dielectric constant (Er) of the coverlay to as high as 4.2 to 4.5 and, when the coverlay is applied to the printed circuit board, may raise the dielectric constant of the whole assembly to undesirable levels thus increasing signal delay time and cross-talk characteristics of the assembly.
The coverlay films described above are generally laminated and molded to the surface of a printed circuit board by application of heat and compressive force, most often in a vessel or container operated under vacuum. As the coverlay is laminated the adhesive flows to fill the spaces between the printed conductors and bonds the coverlay to the printed circuit board. The need for the adhesive to flow can lead to further problems.
In many instances punched or drilled holes are made in the coverlay prior to lamination in order to provide access through the coverlay to the circuitry it covers. It is desirable that these holes be as small as possible, however, during lamination of the coverlay to the printed circuit board the adhesive tends to flow laterally which can result in closure of the access holes. This imposes further limitations on adhesive material selection and on manufacturing processes for their application that can require unwanted compromises that influence conductor spacing, access hole size, materials selection, manufacturing methods, and other factors that can affect the performance and value of the assembly.